A resonant cavity is a device having an enclosed volume bounded by electrically conductive surfaces and in which oscillating electromagnetic fields are sustainable. Resonant cavities may be used as filters, for example, and have excellent power handling capability and low energy losses. Several resonant cavities may be coupled together to achieve sophisticated frequency selective behavior.
Since the geometrical shape of a resonant cavity determines its frequency of resonance, high mechanical accuracy is required and, in addition, or alternatively, post-production tuning is applied. For example, tuning mechanisms may be provided, such as tuning screws that project into the cavity volume by a variable amount and are adjusted manually. During operation, thermal expansion of the component parts of a resonant cavity may occur because of changes in ambient temperature and/or self-heating, leading to frequency deviation. This is usually an unwanted effect and various means exist to compensate for temperature variations.
Resonant cavities are often milled in, or cast from, metal. The frequency of operation determines the size of the cavity required, and, in the microwave range, the size and weight are significant.
One known method for reducing the weight of a cavity is to manufacture it in plastic and cover its surface with a thin metal film. If milling is used to shape the plastic, it can be difficult to achieve sufficient accuracy, and surface roughness may be an issue. Molding is another approach, but the tooling is expensive. Also, plastic material has a potentially higher thermal expansion coefficient than metal, which can result in greater frequency deviation attributable to expansion effects. A resonant cavity manufactured from plastic may also lack robustness compared to a metal one.
The strength of the plastic material might be insufficient for conventional means, such as screw connections, to be used to secure the resonant cavity in position and for connecting input and output transmission means for coupling energy into and out of the cavity. An alternative to the conventional fixing means used with metal cavities is surface mount soldering. However, the unpredictability of solder flow during the process can be detrimental to achieving accurate placement of resonant cavities.
T. J. Mueller, “SMD-type 42 GHz waveguide filter”, Proc. IEEE Intern. Microwave Symp., Philadelphia, 2003, pp. 1089-1092 describes manufacture of a waveguide filter using surface mount soldering in which a U-shaped metal filter part is soldered onto a printed circuit board (PCB), using the board metallization to define one of the waveguide walls.